No they are fine. Ive had StyleXP since version 1.0 (its now v3.19) and they have never slowed down my PC there is an option to turn off StyleXP from using system resources if your PC lacks ram, but even on a PC with low ram as 512mb she will be okay.
As for the Transformation pack thats a risk the user will have to take. It does make changes that could break windows - the creator does warn you before you start. But ive had no problems with it.
Style XP is safer.
There are many Stylexp Themes websites such as this one :
Visual Themes:
http://www.themejunkie.com/browse.php?type=vs
http://www.themejunkie.com/